It took a year of work and required North America’s largest colony of lab-reared sand flies, but NIAID researcher David Sacks, Ph.D., and his colleagues finally proved something they long suspected: the parasites that cause leishmaniasis have sex. The finding is important because scientists may now be able to use hybrid parasites to pinpoint genes responsible for a severe form of leishmaniasis or other traits of interest.
Leishmania parasites spend part of their lifecycle in sand flies and are transmitted to humans and other mammals through the bite of an infected fly. Leishmania multiply chiefly through asexual splitting. However, says Dr. Sacks, circumstantial evidence suggested that they can reproduce sexually as well. Sex, if any, would likely happen inside the sand fly, when the parasites are in the extracellular space outside the insect’s cells. (In humans and other mammals, the parasites reside within individual compartments inside the host’s cells and thus may have limited opportunity to recombine sexually.) But since it is difficult to get healthy parasites to grow and develop properly inside the sand fly, experimental proof of sexual reproduction was hard to uncover.
Dr. Sacks and his team worked with NIAID grantee Stephen Beverley, Ph.D., and others from Washington University School of Medicine in St. Louis. Dr. Beverley created two strains of Leishmania. One strain carried a gene conferring resistance to the antibiotic hygromycin. The other contained a gene that made it resistant to a different antibiotic, nourseothricin. The drug-resistance genes were located on different chromosomes.
In Dr. Sacks’s lab, the strains were mixed with mouse blood and fed to adult sand flies. About two weeks later—after a new generation of parasites had had time to develop—the scientists removed the flies’ midguts along with tens of thousands of parasites inside each one. When exposed to both antibiotics almost all the parasites died. A few, though, survived. Analysis revealed that these rare, doubly antibiotic-resistant parasites were genetic hybrids; they all had inherited two sets of chromosomes—one from the hygromycin-resistant parent and one from the nourseothricin-resistant parent.
Next, Dr. Sacks and his colleagues tested whether hybrid parasites could be transmitted to mammals. They raised sand flies that were co-infected with both parasite strains and allowed the flies to bite mice. The mice developed skin sores, or lesions, typical of leishmaniasis infection. Within the lesions, the investigators found doubly drug-resistant parasites that were genetically similar to the hybrid parasites extracted directly from sand fly midguts. In contrast, when the scientists injected a mixture of both parasite strains directly into mouse ears, they were never able to find any doubly drug-resistant parasites in the resulting lesions. This strongly suggests that doubly drug-resistant parasites are produced through sexual reproduction inside their insect host but the parasites do not recombine sexually in the mouse.
Leishmania sex appears to be very rare both in nature and in the lab, says Dr. Sacks. However, genetic hybrids do arise often enough to act as windows into the spectrum of disease caused by the parasites. For example, Dr. Sacks and his colleagues observed that some hybrids produced lesions in mice that grew at least as fast as the fast-growing neurseothricin-resistant parent strain, while others produced lesions that grew as slowly as the slow-growing hygromycin-resistant parent strain. Thus, the genes controlling growth rate, which can be used as a surrogate marker of disease virulence, appear to be distributed differently in different hybrids. The genetic hybrids can therefore be used to map the locations of the genes responsible for growth rate or other measurable traits, including drug resistance.
Akopyants NA et al. Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector. Science DOI: 10.1126/Science.1169464 (2009).
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Last Updated February 09, 2011